Method for optimizing the power input of a plurality of hysteresis motors connected in parallel

Abstract

A method for optimizing the power input to a plurality of parallel connected hysteresis motors wherein a three phase alternating voltage source jointly supplies the motors with an adjustable operating voltage. The method includes measuring, during the operation of each motor, the phase angle difference between motor voltage and motor current and determining the power factor therefrom for each motor. The effective current value of each motor is measured during operation and is combined with the power factor to obtain the active current for each motor. An active current limit value at which each motor transitions from synchronous to asynchronous operation is measured once and stored for each motor prior to commencement of continuous synchronous operation. After the motors reach synchronous speed at rated voltage, the control voltage is adjusted to reduce the supply voltage to an optimum working point just above the voltage level at which a first one of the parallel connected motors begins to transition from synchronous to asynchronous operation. The active current of each motor is then compared with its associated limit value to determine whether it is operating synchronously or asynchronously. The control voltage of the voltage source is thereafter regulated for increasing the supply voltage to the rated voltage value when it is determined that one of the motors is operating asynchronously and for decreasing the supply voltage to the optimum working point when the motors are all determined to be operating synchronously.

What is claimed is: 
     
       1. A method for optimizing the power input to a plurality of parallel connected hysteresis motors all having the same given rated voltage, wherein a three phase alternating voltage source jointly supplies the motors with operating voltage, the voltage source being adjustable within predetermined limits by means of a control voltage, said method comprising: (a) measuring, during the operation of each parallel connected motor, the phase angle difference between the voltage supplied to the motor and the motor current;   (b) determining the power factor from the measured phase angle difference for each motor;   (c) measuring the effective value of the current of each motor during operation;   (d) combining the respective effective current values and power factors to determine the active current component for each motor during operation;   (e) measuring and storing the limit value of the active current component at which each motor transitions from synchronous to asynchronous operation, said limit value measuring and storing being conducted only once for each motor prior to commencement of continuous synchronous operation;   (f) allowing each motor to reach synchronous speed while maintaining the supply voltage at rated voltage;   (g) while maintaining synchronous speed, adjusting the control voltage of the voltage supply to reduce the voltage jointly supplied to the motors from rated voltage to an optimum working point which is close to the voltage level at which a first one of said parallel connected motors begins to transistion from synchronous to asynchronous operation, whereby the active and reactive power inputs are reduced to a substantially minimum value at the optimum working point;   (h) comparing the active current component of each motor, after said adjusting step, with a respective one of the stored active current component limit values to determine as to each motor whether it is operating with its active current below the stored limit value and therefore operating synchronously or whether it is operating with its active current at or above the stored limit value and therefore operating asynchronously; and   (i) regulating the control voltage of the voltage source for increasing the voltage jointly supplied to the motors to the rated voltage value when said comparing step determines that one of said motors is operating asynchronously and for decreasing the voltage jointly supplied to the motors to the optimum working point when the motors are all determined to be operating synchronously.   
     
     
       2. A method according to claim 1 wherein at the optimum working point said control voltage is a preset desired control voltage, and said regulating step for increasing the supply voltage to rated voltage includes adding a regulating voltage component to the preset desired control voltage such that the supply voltage is increased to rated voltage. 
     
     
       3. A method according to claim 1 further including the step of storing an address identifying each motor and said comparing step further includes the step of providing a signal indicating the address of the motor determined to be operating asynchronously.